James P. O’Neill began his career 33 years ago patrolling the city’s crime-ridden subways. On the day that it was announced he was to succeed William Bratton as NYPD commissioner, O’Neill choked back tears as he remembered his mother’s wisdom.

“She really was the one who taught me the ideals of what good cops should aspire to,” he said, as his mom, Helen, sat in the front row of the City Hall Blue Room for her son’s appointment.

The Tiffany gold and platinum commissioner shield that O’Neill inherits comes with the same mandate that Bratton had: Keep crime rates at historic lows while helping Democratic Mayor Bill de Blasio fulfill a campaign pledge to heal the NYPD’s rift with minorities who have long complained of unfair treatment.

O’Neill, now chief of department, assumes command at a time of national reckoning over race, criminal justice and the use of force.

Speaking at a City Hall news conference to announce Bratton’s resignation after 31 months, O’Neill, 58, said he shares de Blasio’s goal “to help shift this nation’s largest police department away from a style of policing the city that sometimes lost focus.” He said he hopes to keep “lowering crime, but not at the expense of losing the vital support of the people that we are sworn to protect and serve.”

O’Neill became chief — the top uniformed NYPD cop — in 2014, in the nascent days of the Black Lives Matter movement, which protested the police killings of black men. Within weeks, a man believed to have mental problems killed two officers in Brooklyn after vowing on social media that he’d be “putting wings on pigs today.”

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“The protests in the fall of 2014 signaled that change was necessary,” he said of the Black Lives Matter demonstrations. “With the brutal assassination of Joe [Wenjian] Liu and Rafael Ramos in Brooklyn that December, it was clear the NYPD had to evolve and find a new way forward to meet the needs of every New Yorker.”

Under Bratton, O’Neill, who is known as Jimmy, helped design what the de Blasio administration believes is that way forward: a pilot neighborhood policing program. Among the program’s features: a designated precinct officer tasked with fostering neighborhood connections by getting to know locals and becoming a familiar face on the beat. O’Neill cobbled together the program based on ideas from counterparts in Boston, Los Angeles and Seattle.

O’Neill, who was born and raised in Brooklyn’s East Flatbush neighborhood, began became a cop in 1983, joining what was then the New York City Transit Police, a separate force later merged with the NYPD.

Within four years, O’Neill began his ascent through the ranks — to sergeant in 1987, and eventually deputy chief, in 2005. He’s helped supervise firearms training, the academy, the warrant section, and those units dedicated to stopping narcotics and vice, as well as commanding the 25th Precinct in East Harlem, the 44th Precinct in the Bronx and the Central Park Precinct. Bratton made him chief of patrol before he assumed his current job.

O’Neill’s career trajectory had hit a rough patch in 2008, when then-Commissioner Ray Kelly transferred O’Neill out of his post as a narcotics commander after officers under his supervision paid informants not with cash or a break but drugs seized from dealers they had fingered. The practice and its exposure jeopardized dozens of prosecutions.

According to a published report, O’Neill talked to Bratton in 2013 for advice. Bratton advised him to stay on the job and suggested he’d be returning to New York City for a second time as commissioner. O’Neill’s decision paid off Tuesday.

The commissioner’s salary is $219,773, according to de Blasio spokeswoman Monica Klein. O’Neill now lives in Yonkers but under city rules must move into New York City, according to de Blasio spokesman Eric Phillips. O’Neill is divorced with two adult children.

As Bratton was preparing to pass his shield down to his successor inside City Hall Tuesday, activists outside were cheering the resignation — and demanding radical changes to the NYPD.

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“I’m in full support of advocacy groups and everyone’s right to peacefully protest,” O’Neill said.

The protesters outside were not in support of O’Neill: a group now called New Yorkers Against Bratton issued a statement saying they were considering a new name: “We will be ‘New Yorkers Against O’Neill.’ ”